Fix Problem With Contact Form Sending Emails On Joomla

Hi, I have just setup Joomla on a new server. Since I did this my contact form is not working. The form needs to send the details of the contact form to an email address that is hosted on office365. It also sends a confirmation email to the user who filled out the form. Sometimes I can get the form to work, and sometimes It doesn't work. It seems to work with some email addresses but not other email addresses. Maybe there is something wrong with the configuration on the server, or the php settings, or maybe it is a problem with the contact form itself on Joomla. I am not sure whether I should send emails using PHPmail or SendMail, and whether i should use the localhost as an SMTP or use office 365 as an SMTP. If someone could help me get this working it would be great, the form needs to work for all users with email addresses from all around the world. This is a fixed price project. It may be easy for someone, or it may be difficult so please be careful bidding, i have spent a lot of time trying to fix this and I have not been able to. Please only bid if you are an expert on this type of thing. David